1. Field of the Invention
This invention relates to the biological conversion of CO and mixtures of CO2 and H2 to liquid products.
2. Detailed Description
Biofuels production for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels is increasing worldwide. Such biofuels include, for example, ethanol and n-butanol. One of the major drivers for biofuels is their derivation from renewable resources by fermentation and bioprocess technology. Conventionally, biofuels are made from readily fermentable carbohydrates such as sugars and starches. For example, the two primary agricultural crops that are used for conventional bioethanol production are sugarcane (Brazil and other tropical countries) and corn or maize (U.S. and other temperate countries). The availability of agricultural feedstocks that provide readily fermentable carbohydrates is limited because of competition with food and feed production, arable land usage, water availability, and other factors. Consequently, lignocellulosic feedstocks such as forest residues, trees from plantations, straws, grasses and other agricultural residues may become viable feedstocks for biofuel production. However, the very heterogeneous nature of lignocellulosic materials that enables them to provide the mechanical support structure of the plants and trees makes them inherently recalcitrant to bioconversion. Also, these materials predominantly contain three separate classes of components as building blocks: cellulose (C6 sugar polymers), hemicellulose (various C5 and C6 sugar polymers), and lignin (aromatic and ether linked hetero polymers).
For example, breaking down these recalcitrant structures to provide fermentable sugars for bioconversion to ethanol typically requires pretreatment steps together with chemical/enzymatic hydrolysis. Furthermore, conventional yeasts are unable to ferment the C5 sugars to ethanol and lignin components are completely unfermentable by such organisms. Often lignin accounts for 25 to 30% of the mass content and 35 to 45% of the chemical energy content of lignocellulosic biomass. For all of these reasons, processes based on a pretreatment/hydrolysis/fermentation path for conversion of lignocellulose biomass to ethanol, for example, are inherently difficult and often uneconomical multi-step and multi conversion processes.
An alternative technology path is to convert lignocellulosic biomass to syngas (also known as synthesis gas, primarily a mix of CO, H2 and CO2 with other components such as CH4, N2, NH3, H2S and other trace gases) and then ferment this gas with anaerobic microorganisms to produce biofuels such as ethanol, n-butanol or chemicals such as acetic acid, butyric acid and the like. This path can be inherently more efficient than the pretreatment/hydrolysis/fermentation path because the gasification step can convert all of the components to syngas with good efficiency (e.g., greater than 75%), and some strains of anaerobic microorganisms can convert syngas to ethanol, n-butanol or other chemicals with high (e.g., greater than 90% of theoretical) efficiency. Moreover, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, peat, petroleum coke, coal, solid waste and land fill gas, making this a more universal technology path.
However, this technology path requires that the syngas components CO and H2 be efficiently and economically dissolved in the aqueous medium and transferred to anaerobic microorganisms that convert them to the desired products. And very large quantities of these gases are required. For example, the theoretical equations for CO or H2 to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2O
Thus, 6 moles of relatively insoluble gases such as CO or H2 have to transfer to an aqueous medium for each mole of ethanol. Other products such as acetic acid and n-butanol have similar large stoichiometric requirements for the gases. Furthermore, the anaerobic microorganisms that bring about these bioconversions generate very little metabolic energy from these bioconversions. Consequently they grow very slowly and often continue the conversions during the non-growth phase of their life cycle to gain metabolic energy for their maintenance. To get high yields and production rates the cell concentrations in the bioreactor need to be high and this requires some form of cell recycle or retention.
Cell retention by formation of biofilms is a very good and often inexpensive way to increase the density of microorganisms in bioreactors. This requires a solid matrix with large surface area for the microorganisms to colonize and form a biofilm that contains the metabolizing microorganisms in a matrix of biopolymers that the microorganisms generate. Trickle bed and some fluidized bed bioreactors make use of biofilms to retain microorganisms on solid surfaces while providing dissolved gases in the liquid by flow past the solid matrix. They suffer from either being very large or unable to provide sufficient gas dissolution rates.
Particular forms of membranes have found use in supporting specific types of microorganisms for waste water treatment processes. U.S. Pat. No. 4,181,604 discloses the use of hollow fiber membranes for waste treatment where the outer surface of the fibers supports a layer of microorganisms for aerobic digestion of sludge.
US 2008/0305540 discloses a membrane based bioreactor wherein anaerobic bacteria that have the ability to convert syngas to ethanol or other liquids have formed biofilms on the outer surface of hydrophobic membranes with the syngas fed to the bacterial biofilm through the inner surface of the membrane. Such a bioreactor system has been able to directly convert the primary components of synthesis gas, CO and H2/CO2, to ethanol and other liquid products such as n-butanol, acetic acid and butyric acid. In these systems the gas flows through a porous region of a hydrophobic membrane and then reaches a biofilm which is hydrophilic. One drawback of this arrangement is that if water reaches and deposits/condenses on the hydrophobic porous region it will severely decrease the gas transfer rate. Since the biofilm grows on the outside of the membrane, this type of membrane system also lacks a direct means to promote the formation of a biofilm with an adequate thickness and control its performance.
Asymmetric membranes are known for use in a variety of membrane separations processes such as ultra and nano filtration. Asymmetric membranes are typically hydrophilic and have a relatively tight semi permeable “skin” layer on one side supported on a porous “spongy” polymer layer. U.S. Pat. Nos. 4,442,206 and 4,440,853 show the use of the “spongy” polymer layer in an asymmetric membrane to immobilize microorganisms for certain biological processes that use soluble carbon sources. However, the adaptation and use of such membranes for the anaerobic bioconversion of syngas to liquids has not been shown in the past.